Citric acid is widely used in the food and beverage industry, pharmaceutical industry and elsewhere. World-wide demand for this compound is in excess of 200,000 metric tons a year. Historically, and even today, the chief processes for the production of citric acid are microbial fermentation processes. A large number of organisms including fungi, yeast and bacteria have been examined as microbial agents to produce citric acid.
Aspergillus niger employed since the early part of this century is still apparently the organism of choice.
The basic methodology for citric acid production is throughly reviewed in "Prescott & Dunn's Industrial Microbiology" 4th Ed., 1982, Reed, edit., AVI Pub. Co., Inc., Westport, Conn., pps. 709-747 (hereby incorporated by reference).
Three principal methods of citric acid production by microbes are known: solid state culture, continuous culture and multi-stage fermentation processes. Most new technology is apparently based on submerged fermentation processes. The usual carbon sources are sucrose or molasses.
Where solid state fermentation has been employed the fermentation medium has been impregnated in porous solid materials such as sugarcane bagasse, potato or beet pulp, pineapple pulp and the like and then inoculated.
Apple pomace is the residue left from juice extraction and constitutes about 25% of the weight of fresh fruit (Walter et al., 1976, J. Agr. Food Chem., 24: 1244-1245). Approximately 1.3 million metric tons of apple pomace are produced annually in the United States (Jewell et al., 1984, J. Food Sci., 49: 407-410) at present apple pomace is disposed of by trucking to land for application to the soil. This procedure is costly ($10/$20/ton) and also presents environmental problems. Apple pomace is rich in carbohydrate, but its protein content is low (Bomben et al., 1971, Food Technology, 25: 1108-1117). Significant research effort has been made to use apple pomace as an energy source, alleviating the waste disposal problem (Walter et al., supra; Jewell et al., supra).